Getting the Biggest Bang for Your Air-Sealing Buck

If you don’t have time to seal every last crack, which cracks should you seal first?

The five most important areas for air sealing. If you have limited resources to devote to air sealing, focus on recessed can lights, ceiling-mounted duct boots, the cracks at partition top plates, walls between the house and an attached garage, and rim joists. [Click on the image to enlarge it.]

Most new homes are leaky. In the typical new home, significant volumes of air enter through cracks near the basement rim joists and exit through ceiling holes on the building’s top floor. These air leaks waste tremendous amount of energy.

Of course, some builders have focused on energy efficiency for years, and many of these builders own a blower door. If you have your own blower door, you have probably learned by trial and error which cracks matter most.

However, the vast majority of contractors build homes without any feedback from a blower door. If these builders want to improve the airtightness of the homes they build, they probably don’t know where to start.

Get the big holes first

The first step is to make sure that there aren’t any really big holes in your homes. (Joe Lstiburek calls these “the Joe-sized holes”; they’re the holes that are big enough for Joe to crawl through.) You may be thinking, “Can a house really have holes that big?” The answer, sadly, is “Yes, it really can.”

Let’s raise the bar just a little, and make a list of holes that are big enough for a cat to walk through. These include:

Holes in the air barrier behind zero-clearance metal fireplaces.

Unsealed holes above kitchen soffits.

Unsealed holes above dropped ceilings.

Attic access hatches or pull-down attic stairs without any weatherstripping.

Unsealed utility chases that connect basements with attics.

Holes behind bathtubs installed on exterior walls.

Once these holes are patched — in most cases, using OSB, plywood, rigid foam, or ThermoPly — what’s next? If you are a PassivhausA residential building construction standard requiring very low levels of air leakage, very high levels of insulation, and windows with a very low U-factor. Developed in the early 1990s by Bo Adamson and Wolfgang Feist, the standard is now promoted by the Passivhaus Institut in Darmstadt, Germany. To meet the standard, a home must have an infiltration rate no greater than 0.60 AC/H @ 50 pascals, a maximum annual heating energy use of 15 kWh per square meter (4,755 Btu per square foot), a maximum annual cooling energy use of 15 kWh per square meter (1.39 kWh per square foot), and maximum source energy use for all purposes of 120 kWh per square meter (11.1 kWh per square foot). The standard recommends, but does not require, a maximum design heating load of 10 W per square meter and windows with a maximum U-factor of 0.14. The Passivhaus standard was developed for buildings in central and northern Europe; efforts are underway to clarify the best techniques to achieve the standard for buildings in hot climates. builder aiming to achieve 0.6 ach50, the answer is simple: every conceivable crack in the home’s thermal barrier needs to be sealed. In some cases, Passivhaus builders use a redundant approach — for example, using both caulk and a gasket.

If you are a production builder, you probably don’t have the time or inclination to approach air sealing with a fastidious attention to detail that Passivhaus builders employ. So perhaps you buy a case of caulk and begin by sealing the cracks between double studs and double top plates. Or maybe you focus on sealing the cracks around windows. When you run out of caulk, you might call it a day.

Does this approach make sense? Not really.

Quantifying the results of sealing measures

Dave Wolf, a senior research and development project leader at Owens Corning, has completed a study to determine which cracks and holes result in the “biggest bang for your air-sealing buck.” Wolf’s research had two components: laboratory measurements of air leakage through several 8 ft. by 8 ft. mockups of building assemblies, and field research at a 1,400-square-foot Owens Corning test house. (To measure the results of different air-sealing measures, the researchers used a blower door: “All joints were selectively sealed and/or unsealed for measuring their contribution to the overall air leakage of the house.”)

Wolf concluded that the five most important areas for builders to focus their air-sealing efforts are:

Cracks at recessed can lights in the top-floor ceiling.

Cracks between duct boots in the top-floor ceiling and the ceiling drywall.

Cracks between the top plates of top-floor partitions and the partition drywall.

Leakage through walls separating a house from an attached garage.

Cracks in the rim-joist area.

A few comments on Wolf's findings:

The researchers did not test leaks around floor-mounted duct boots.

The researchers did not test leaks at cracks between ceiling drywall and bath exhaust fan housings. Wolf speculates that these leaks might be worse than the leaks around ceiling-mounted duct boots, because with most bath fans, "There isn’t necessarily a flange. Unlike with a duct boot, it is a flangeless opening."

Since leakage was measured by a blower door, the reported results exaggerate the importance of leaks near the neutral pressure plane, and didn't properly evaluate the way the stack effectAlso referred to as the chimney effect, this is one of three primary forces that drives air leakage in buildings. When warm air is in a column (such as a building), its buoyancy pulls colder air in low in buildings as the buoyant air exerts pressure to escape out the top. The pressure of stack effect is proportional to the height of the column of air and the temperature difference between the air in the column and ambient air. Stack effect is much stronger in cold climates during the heating season than in hot climates during the cooling season. disproportionately depressurizes the lowest areas of a house and pressurizes the highest areas of a house. Is spite of this fact, four out of five of the highlighted areas are either down low (the basement rim joist) or up high (recessed cans, ceiling duct boots, and top-plate cracks). My conclusion: these leakage areas are even more important than this research indicates. So be sure to seal these areas!

Walls between a house and an attached garage made the list due to the fact that these walls leak more than other walls — not due to any concerns over indoor air quality, nor to the fact that air in a garage is often contaminated. Why are these walls leaky? “These are the only exterior walls where you have drywall on both sides,” Wolf told me. “Drywall is a flimsy material compared to OSB sheathingMaterial, usually plywood or oriented strand board (OSB), but sometimes wooden boards, installed on the exterior of wall studs, rafters, or roof trusses; siding or roofing installed on the sheathing—sometimes over strapping to create a rainscreen.
or plywood. When you mechanically fasten drywall on the outside of the studs, the crack where it mates with the framing is not as tight as what you get with OSB or plywood.” These walls are good candidates for the Airtight Drywall Approach.

At the opposite end of the spectrum are the cracks that take a lot of sealant and a lot of time to seal, without reducing total air leakage by very much. This group — let’s call it the “why bother?” list — includes vertical sheathing joints, the cracks between double top plates, and the cracks between the wall sheathing and the framing of window rough openings.

Air-sealing tips

For more information on ways to seal air leaks in these areas, check out the resources listed below.

Recessed Can Lights: “In most cases, these innocent-looking circles are actually holes in your ceiling. Not only do recessed can lights leak air, but warm lightbulbs also make the situation worse, turning the holes into small chimneys. The heat source accelerates the stack effectAlso referred to as the chimney effect, this is one of three primary forces that drives air leakage in buildings. When warm air is in a column (such as a building), its buoyancy pulls colder air in low in buildings as the buoyant air exerts pressure to escape out the top. The pressure of stack effect is proportional to the height of the column of air and the temperature difference between the air in the column and ambient air. Stack effect is much stronger in cold climates during the heating season than in hot climates during the cooling season., speeding up the flow of air.”

Duct boots. “Practically no sealing takes place [at duct boots]. … It’s also not just a single issue at the boots, it’s two specific locations: between the boot and the metal frame and more importantly, between the ceiling drywall (or subfloor) and the metal frame. … Why this is not the HVAC contractors fault is the timing of construction activities. The time to perform this sealing usually doesn’t align with their typical site visits. Should it be the painters? Drywallers? Insulators? Site supervisors? Punch out specialists? I think the answers will vary, and it will be up to the individual builders to decide when this sealing should take place given their particular build process.”

Cracks between partition top plates and drywall. “Between the drywall and the top plate of the wall, we have a few different choices to seal up. I tend to go for an acoustical sealant. The stuff is very sticky, and it doesn’t completely cure. It will stay sticky and be able to move with the materials.”

Rim joists: “The next step was to insulate and air seal at the rim joist, so that the thermal boundary was continuous to the bottom of the subfloor above. I chose to do this with a two-component spray polyurethane kit.”

A few caveats

The study provides a “blower‐door‐centric point of view. All of the … results are prioritized based on the effect on whole‐house leakage, not thermal comfort, IAQIndoor air quality. Healthfulness of an interior environment; IAQ is affected by such factors as moisture and mold, emissions of volatile organic compounds from paints and finishes, formaldehyde emissions from cabinets, and ventilation effectiveness., etc.”

It best to try to “seal all the joints, if you can. The sealing of all joints/openings is important, although some are more important than others.”

It makes sense to “get the big holes first. This study focuses on small joints/openings only (i.e., the big holes are presumed to be blocked & sealed).”

“Sometimes the claddingMaterials used on the roof and walls to enclose a house, providing protection against weather.
matters. The wall cladding is assumed to be air permeable (e.g., vinylCommon term for polyvinyl chloride (PVC). In chemistry, vinyl refers to a carbon-and-hydrogen group (H2C=CH–) that attaches to another functional group, such as chlorine (vinyl chloride) or acetate (vinyl acetate)., fiber-cement, wood siding, or brick, not stucco or stone veneer).”

“These results are for general guidance. These results should be considered directional, not absolute, since construction quality varies from house to house.”

“Don’t abandon common sense. If you can see daylight through a joint, it should be sealed, regardless of what this study may indicate.”

Drywall is the unsung hero of air-sealing efforts

When weatherization contractors try to seal leaks in an existing home, the work is known as “blower-door-directed air sealing.” Veterans of this type of work know that some time-consuming measures have little effect on a home’s air leakage rate, while other simple measures produce good results. Here are two lessons that weatherization contractors have learned:

It always makes sense to seal the big holes first.

Holes near the bottom of the house (in a crawl space or basement) and holes near the top of the house (in the ceiling of the top floor) matter much more than holes near the center of the house (an area that is also known as the "neutral pressure plane").

Dave Wolf’s research looks at these issues from the perspective of a new home builder rather than a weatherization contractor. His results give important guidance to builders who are just beginning to think about air sealing.

The research results correlate well with long-standing advice on air-sealing. “When you stack up these results with typical air-sealing advice, the list passes the gut check for what has been said for a long time,” Wolf told me. “That might be deflating, since we just found out what the industry already knew. But what is novel about this is that we have quantified the amount of leakage per unit length for all of these joints. We know for a top-plate-to-drywall crack how many cfm50 per foot that the crack leaks. And we can take those parameters to do calculations: ‘Is it worth it to me to seal this particular joint?’ With the information we have, you can calculate that if you have 100 feet of this type of joint, the sealing ought to be worth about so much to me from a blower-door standpoint. We can use the information to help make strategic decisions.”

Wolf's research demonstrated the contribution that drywall makes to the airtightness of wall assemblies. “When we tested naked walls without drywall in place, they leaked a lot,” Wolf told me. “Sometimes you could even see daylight through the cracks. But when you put the drywall on, the leakage went down dramatically. The drywall is providing a secondary air barrier. There might be five different locations in the wall cavity where air is leaking in, but the air is having a hard time getting past the drywall. The pressure in the wall cavity builds up, and you don’t have a 50 pascal pressure difference across your sheathing; you do across the drywall, but not across the sheathing. It took making the measurements for me to see the effect of this traffic jam in the wall cavity.”

If you care about airtightness, it's probably time for you to bring your painter a box of donuts. “When the painter comes along, the builder holds the painter accountable,” said Wolf. “The painter has to make sure that finishes look magnificent, and the work includes caulking the trim pieces to the drywall. What the painter is inadvertently doing is improving the airtightness of the drywall layer. We would never advocate using painter's caulk as part of your air barrier. It is not the right approach. But it is interesting that it does have an impact. The painter is playing a role in improving the airtightness of the wall.”

If you are foaming the underside of a roof to create a sealed attic, are can lights okay? Would you still want to (a) substitute a different type of light fixture or (b) create an air seal around each fixture? I think I know the answer, but my brain is a bit overloaded with business sciences info.

2.
Aug 23, 2013 10:52 AM ET

Edited Aug 23, 2013 11:39 AM ET.

Cold Climate Bias ? by John Brooks

Martin wrote: "the stack effect disproportionately depressurizes the lowest areas of a house and pressurizes the highest areas of a house."

Martin, how about in a Not-So-Cold Climate?

How about in an Air Conditioned House?
Aren't the highest areas often "depressurized" With Respect To Outside (WRTO)?

Edit to say:
The first paragraph of this article also suggests a Heated Home...
or a Cold Climate Perspective.

Steven,
If you are installing spray foam on the underside of your roof sheathing, you need to make sure that the foam insulation is thick enough to at least meet minimum code requirements for R-value. This should be obvious; but, believe it or not, many spray foam contractors convince their customers that foam is such a miracle product that it's OK to install it in a layer that is too thin to meet minimum code requirements.

Once you've done that, you could install a recessed can light under the spray foam, as long as you meet the clearance requirements established by the recessed can manufacturer. Remember, though, that in many cases the rafters aren't deep enough to accommodate both a recessed can fixture and enough insulation to meet minimum code requirements.

If the spray foam contractor is conscientious, the spray foam will create an air barrier as well as an insulation layer. If you install a recessed can light on the interior side of the spray foam, you don't have to worry about air leaks through the recessed can.

John,
Thanks for the reminder about my bias. Sometimes the need for a fluent writing style forces a writer to be less than complete from a technical standpoint. If a sentence is overloaded with caveats and prepositional phrases, it often becomes so leaden that it isn't worth reading.

Even Texans do experience winter, however, and most need to heat their homes for a few weeks every year.

Because the indoor-to-outdoor delta-T in Texas isn't as large during the summer as the delta-T in Vermont during the winter -- or even, for that matter, as the delta-T in Texas on a cold winter day -- the "reverse stack effect" attributable to air conditioning is less pronounced than the stack effect that occurs during the heating season. In fact, infiltration and exfiltration locations are just as likely to be influenced by wind, exhaust fans, or HVAC blower operation as they are to be influenced by the "reverse stack effect" in Texas during the summer.

That said, I plead guilty to occasional eruptions of cold-climate bias.

5.
Aug 23, 2013 2:31 PM ET

The stack effect does indeed work in both directions. by Dana Dorsett

Just because it's a cooling dominated climate doesn't invalidate the importance of both high & low leakage points, unless you can somehow demonstrate that cold air leaking out the foundation sill sucking hot outdoor air (or worse- ATTIC air) into the top of the house isn't a comfort or energy use problem.

Near the shore in San Diego maybe it doesn't matter too much, but anyplace with more significant heating or cooling loads stack effect pressures really do matter.

But let's get over the climate-bias paranoia, eh? For all but US climate zone 1 locations heating season stack effect infiltration exceeds cooling season stack effects. Even if peak cooling loads may be higher than the heating loads, the seasonal average delta-T drivers of stack effect (air conditioned or not), are greater for during the heating season. Cooling loads are dominated by solar gain, whereas stack effect is driven by air temperature deltas. The effect of infiltration rates on heating & cooling loads are also a function of air temperature deltas. Even in TX and most of FL there's a bigger net effect and energy use from stack effect infiltration during the heating season than the cooling season. (Only down in Brownsville TX is it close to a wash, with roughly equal heating/cooling season stack effect infiltration.) A heating-season bias is appropriate when addressing this issue, more so than with other issues.

If you live in Miami or Hawaii you can pretty much ignore the heating-season stack effect discussion, but it's valid for the rest of the US.

6.
Aug 25, 2013 7:27 AM ET

Edited Aug 25, 2013 7:32 AM ET.

I might catch Hades from some other Pros... by Curt Kinder

Bear with me a couple minutes while I argue for the evil spray foam guys:

If I'm starting out with an existing home with attic ductwork, I'd rather have an air tight code minimum (here R19) foam job than R 30-40-60 anything of loose fluffy stuff.

Sure, the engineer in me would rather have R38 or better spray foam, but the payback on the 'second' R19 is vanishingly small, on the order of a century.

In fact, given the unit costs of spray foam, there is probably a valid case to be made for dropping BELOW code minimum insulation levels, IF the project is PROVEN air tight.

Can lights seal the deal - houses with lots of them derive the most benefit from retrofitted foam.

No one likes to give up so much as a scintilla of R value, but economic reality suggests a blended solution of moderate R-Value and heavy air sealing, at least down to 2-3ACH50.

I repeat that the foam job must be proven as air tight as reasonably possible - a loose foam job can do more harm than good by limiting sensible load while leaving latent load. No good comes of that, occupants are unhappy, and foam's reputation suffers. Therefore we don't participate in a foam project if we aren't engaged to control quality.

7.
Aug 25, 2013 10:54 AM ET

Do these results depend on where the air barrier is located? by Jerry Chwang

I'm probably not fully understanding this, but isn't the location of the designed air barrier important to prioritizing what needs to be sealed? e.g. I'm going with the REMOTE approach with air barrier on the outside face of the sheathing, including over a flat roof.

Doesn't that make anything on those surfaces critical for air sealing, e.g. Vertical seams (identified in the research as low priority) in the plywood should be sealed because the plywood is covering everything, including the band joists (high priority).

Related to this, how important is the drywall in this case?

Of is the research identifying weak points, but you can address in different ways? including redundant layers, e.g. caulking band joist and exterior air barrier?

Curt,
I understand your point. Spray foam has certain advantages, including the potential for greatly improving airtightness. I suggest that we should be aiming to achieve airtightness as well as (at least) code-minimum R-values.

Spray foam contractors whine, "But spray foam is so expensive that it is hard to compete with cellulose!" True. But that might be a good argument in favor of cellulose.

In your area, where HVAC contractors mysteriously put their ductwork and air handlers outdoors (in the attic), you have an additional problem. If you are building an unvented conditioned attic, you have fewer options for insulation. But if spray foam is too expensive, you can choose to use rigid foam above the roof sheathing, or a combination of rigid foam and air-permeable insulation, or a combination of spray foam and air-permeable insulation.

Jerry,
Q. "Do these results depend on where the air barrier is located?"

A. As far as I understand it, the Owens Corning researchers were looking at typical American construction techniques. They were assuming that builders are just beginning to consider air sealing, for the first time in their careers.

If you ask the typical American framer, "Where is the air barrier?" he will have one of two answers. He will either say, "What's an air barrier?" or "You mean the housewrap?"

The houses that the Owens Corning researchers looked at had no air barrier. Before they began their iterative process of air sealing various cracks, the homes leaked. There were cracks between the sheathing panels. There were cracks near the windows and doors. The ceilings and rim joists leaked in many, many areas. The drywall layer slowed down, but did not stop, these leaks.

For those who have been paying attention to air sealing for decades, it may seem surprising to learn that the average American home builder doesn't know where his air barrier is located. But the sad fact is, he doesn't.

Q. "I'm going with the REMOTE approach. ... Doesn't that make anything on those surfaces critical for air sealing, e.g. vertical seams ... in the plywood?"

A. Yes. That is why David Wolf wrote that “it best to try to seal all the joints, if you can. The sealing of all joints/openings is important, although some are more important than others.”

Q. "How important is the drywall in this case?"

A. If you are building a REMOTE or PERSIST house, the air barrier is created at the exterior wall sheathing, which means that it is on the interior side of the insulation. In such a house, there is no need to create a second air barrier at the drywall layer.

Q. "Can you address [air sealing] in different ways? including redundant layers, e.g. caulking band joist and exterior air barrier?"

A. Of course. That is why I wrote, "If you are a Passivhaus builder aiming to achieve 0.6 ach50, ... every conceivable crack in the home’s thermal barrier needs to be sealed. In some cases, Passivhaus builders use a redundant approach — for example, using both caulk and a gasket."

11.
Aug 28, 2013 9:08 PM ET

Edited Aug 28, 2013 9:11 PM ET.

plaster value by john roy

Martin
If painter's caulk provides some value in stopping air flow does blueboard finished with thin coat plaster provide any value in sealing the wallboard?

Roy,
Plaster is a good air barrier (equivalent to taped drywall). As with drywall, air leakage won't occur in the center of a plaster wall. Likely air leakage points occur at electrical boxes, penetrations, window rough openings, door rough openings, under bottom plates, and between the top plate and the blueboard.

These areas need to be sealed in the same way they would if you were following the Airtight Drywall Approach.

13.
Aug 29, 2013 8:55 AM ET

Attic kneewalls by Mark Newey

I would think attic kneewalls would make the "biggest bang for the buck" list - like garage walls, they often have something other than standard OSB sheathing on the exterior side and can leak quite a bit.

14.
Aug 30, 2013 10:22 AM ET

Can Lights by Richard James

Martin,
Can lighting is a necessity in many homes, esthetically and practically. My house has a dozen of them in kitchen and living room. I read, perhaps in GBA, that each can leaks 4 CFM and I don't doubt that. Plus my cans are not IC-rated so the attic insulation is pulled back at the housings. The advent of LED and the rapidly dropping cost of same offers solutions on a few levels. I bought Eco-Smart Cree-technology LED can inserts (dimmable) on sale at my local big box store. My original intention was to install air tight baffles at about $8 each but stumbled onto to the LED's instead. Last time I checked they were $50 each but price has dropped by half. We love them. Higher quality light than incandescent spots and only 10w each (versus 65). The old leaky cans were (I hope) an easy solution. I stuck on some white foam rubber 3/8"w x 3/16"thk weather strip on the perimeter of the baffle to seal the ceiling joint. Rubber foam is more pliable than vinyl and compresses better on this low pressure joint. I want to thank GBA for getting me on this mission. I have sealed my attic hatch with weather strip and found a massive 1/2 inch gap around the stove vent duct penetration. It was so bad there were dust tracks on the cabinet door joint. Dust/dirt tracks are a good indicator, which I learned from my AC tech. How did I miss that all of these years? Sealed it with expanding foam. Also found many leaks in skylight joints. Will be hitting crawlspace and attic to seal plumbing and electric leaks before winter. Looking forward to propane savings this winter.

Richard,
I'm glad that you are making progress in your air sealing efforts. Keep up the good work.

According to some observers, can lights may be aesthetically pleasing. Others, including me, recoil in horror when they see one. So your statement that "Can lighting is a necessity in many homes, esthetically and practically" seems indefensible to me. A more accurate statement would be that "some homeowners find can lights to be attractive." There is no accounting for taste. It's your house.

Just be aware of the energy penalty you are paying for you aesthetic preference.

Martin, No choice in my house, they were already there sucking $$ propane heated air up info the attic. I'll rephrase, the Eco-Smart inserts are more attractive than the originals. The kitchen group brings light to all corners, they are great. The LED's dipserse light far better than the uncandescents. I could do without the living room sets. Rarely use them. Great site, glad I found it. Never would have learned about "stack effect". My wife thinks I've gone nuts since I retired. Attic fanatic. Idle minds...

17.
Aug 30, 2013 11:54 PM ET

Redundency VS Perfection by Russell Higgins

Not a builder, just grew up with them - went on to become an architect, jury is still out on that.

What I experienced growing up, and helping out, builders, to this day when pros help me out on my DIY farmhouse rebuild, is that SPEED is the name of the game.

Given the need for speed, more on that below, and the generally low QC on low bid (most) jobs where day workers outnumber permanent staff, language issues exist, etc. (see below), I can't help but believe the only solution to air / moisture barriers is redundency, perferably different trades. Say seal sheathing seams and tape house wrap along with airtight drywall AND a list of must seals as noted in the article.

Site mockups help, it provides a "training sample" for day workers and bridges the language gap,. After framing pick a strip top to bottom, have GC build it out, maybe without all the siding.

The need for speed: Can't fault builders for working fast. Lots of builders with good references, so it boils down to price
The need for quality: NYC is putting quality back in construction. Large buildings report yrly energy use, result, . Much publicized BA "LEED Platinum" building is actually a energy hog. The tenants are being blamed, long story and the jury is still out. None the less, the architect, engineers, buidler, and developer have all lost credibility, and no doubt customers, due to the under performance of the building. NYC is also requring certified inspections of at least 10% of all energy related materials and assemblies.

Russell,
You raise an important point: if builders work fast, they may not care about air sealing. That's true.

There is a simple remedy to this problem: put an airtightness goal in the specifications, and hold the GC accountable for reaching the target. If everyone on the job site knows that the job will be tested with a blower door, and if payment depends on hitting the airtightness spec, workers will pay attention to airtightness.

19.
Aug 31, 2013 9:43 AM ET

Air Tightness by Richard James

Layperson here and very interested. I remember the cheap energy days when a house was supposed to "breathe", I grew up in one of those ice boxes. The envelope deficiencies I am finding in my house, built in 2000, are significant. Custom built, the original owner was there every day. What are they getting away with in spec built subdivisions? My house met code at the time, or should have, so the fault is with the code and the inspection process. It needs to be updated (blower door test) and enforced. The average person has no clue, we have to rely on the builders and a sensible regulatory process. This is not rocket science. $500 extra in materials/workmanship in 2000 would saved thousands in heating and air conditioning. Multiply that by millions of homes and do the math.

20.
Sep 1, 2013 4:39 PM ET

Can we quantify "Cost per hole" by Roger Williams

We have sealed our house at all the "available" joints and holes. There are lots of holes that are not available because we have no attic access. None of the holes in top plates to feed electric wires are sealed within the walls. I have found two while renovating a closet and they are .75 inch holes for each wire. I am contemplating trying to open up some of the walls to seal some of these, but is it worth the effort? The wall by the distribution panel may be worth the effort, but my wife is tired of the cleanup after each attempt.

Roger,
Here in New England, we have a lot of skilled weatherization workers who know how to perform blower-door-directed air sealing in existing homes. Unfortunately, these skilled workers may be lacking in other areas.

When there is evidence of "thermal bypass" leaks through interior partitions -- often due to electrical penetrations that are hard to access, as you describe -- these workers often install dense-packed cellulose in the stud bays of the interior partitions. The cellulose in this case is not working as an insulation; it is working to limit the flow of air through all those holes in the plates.

22.
Feb 10, 2014 10:44 AM ET

Edited Feb 10, 2014 10:46 AM ET.

Leaks! Air Sealing! by Robert Nachtrieb

We live in a 100 year old foursquare: two stories plus basement and attic. Major chimney effect potential. We've had blower tests, blown cellulose into the balloon frame exterior walls, packed cellulose between the joists in the attic floor after sealing all holes and cracks we could find, rebuilt double hung sash windows to tighten up the rails and seal the edges. Put a nice dent in the air exchange, but not enough, and having a hard time finding the rest of the leaks.

Reading this article, it hit me: the exhaust fan in the upstairs bathroom! Nice, expensive Broan, vented through the attic directly to the roof, with a vent cap with a flap valve. Run a blower test and the flap is sucked shut. No problem detected. But live life normally and convection pushes that danged little flap open all winter!

Is there such a thing as a powered vent cap that will open only when the exhaust fan is turned on?

Robert,
Q. "Is there such a thing as a powered vent cap that will open only when the exhaust fan is turned on?"

A. No -- not as far as I know. However, some bath exhaust fans have better backflow dampers than others. I suggest that you post a question on GBA's Q&A page asking GBA readers to suggest the best bath fan in this regard.

About the Author

Martin Holladay has worked as a plumbing wholesale counterperson, roofer, remodeler, and builder. He built his first passive solar house in northern Vermont in 1974, and has lived off the grid since 1975. In 1980, Holladay bought his first photovoltaic(PV) Generation of electricity directly from sunlight. A photovoltaic cell has no moving parts; electrons are energized by sunlight and result in current flow. module, which is still producing electricity after all these years. Read more...